This invention relates generally to the control of machine tools and the like, and more particularly to a method and apparatus for monitoring and controlling a single point machining operation to optimize productivity.
A significant limiting factor on productivity in single point machining operations, such as metal cutting using a lathe, is regenerative instability, which is a major source of chatter in machine tools. Regeneration is the phenomenon whereby past events are fed back with a time delay to drive a system at a later time. Regenerative vibrations often occur in a machine tool when the cutting tool passes over a previously machined area, and are caused by variations in depth of cut and chip thickness due to the normal relative vibrations between the tool and the workpiece. During the single point machining of a rotating workpiece, lobes or waves are formed on the workpiece as a result of these normal vibrations. Lobes move relative to the workpiece during machining, i.e., regenerate, whereby a workpiece lobe which is cut by the tool on one revolution is fed back and drives the tool during the next revolution. If in this process the lobe grows and gets larger, the system is regeneratively unstable; if the lobe gets smaller, the system is regeneratively stable. The regeneration of the lobes causes oscillatory vibrations and chatter, which lead to unacceptable scalloping of the workpiece surface, accelerated tool wear and breakage, and may cause damage to the machine tool.
Regenerative chatter greatly limits the productivity of machine tools. The mechanisms which govern regenerative chatter are not well understood, and this has hampered the development of adaptive control strategies which are effective for minimizing chatter and optimizing cutting and productivity. Because of the dynamic nature of the machining process, it is not possible to establish fixed operating conditions which insure both that the process remains stable and that productivity is optimized. In most cases, regenerative chatter is caused by workpiece flexibility so that modifications to the machine tool structure are ineffective for controlling chatter. In order to maintain optimum cutting and productivity, it is necessary to monitor the machining process and adaptively control its operating conditions. Optimum cutting and productivity can best be obtained by sensing the state of stability of the process and, through suitable controls, by adjusting the machining conditions to maintain optimal control. This in turn requires a better understanding of the dynamic and complex nature of regenerative chatter and of mechanisms which influence chatter so that appropriate stability sensors and an appropriate control strategy can be employed.
It is desirable to provide a method and an apparatus for dynamically monitoring and controlling a single point machining operation to minimize chatter and to optimize the machining operation, and it is to these ends that the present invention is directed.